We report the observation of Fano-like antiresonance lineshapes for ungerade modes in the low temperature spectra of Cu(DM-DCNQI)2 and its deuterated analog. Consideration of the existing structural and optical data leads us to conclude that the unusual vibronic coupling mechanism responsible for the observed lineshapes occurs via the effect of the vibrations on the ppi-d hybridization of the conduction bands. This brings further support to the existence of a mixed-valence Cu(I/II) state in the studied compounds. The observation of additional vibronic effects attributed to electron charge oscillations induced by the a(g) modes in Cu(DM-DCNQI-d8)2 suggests that the deuterium substitution favours the crossing of the border between the low temperature metallic and insulating phases.